Ceramic arc tubes for high-intensity discharge (HID) lamps are well known. One of the more common configurations of these arc tubes includes an axially symmetric discharge vessel having opposed capillary tubes extending outwardly from each end. These capillary tubes have an electrode assembly sealed therein to provide the electrical energy needed to strike an arc discharge inside the discharge vessel. The ends of the capillaries are sealed hermetically to the electrode assemblies with a frit material. The discharge vessel contains an ionizable fill material which usually comprises some combination of metal halide salts and/or mercury. A buffer gas is added to promote arc ignition and influence the lamp's photometric properties and longevity. The typical buffer gas is one of the noble gases, e.g., argon, xenon, krypton, or a mixture thereof. Generally, the buffer gas pressures of ceramic arc tubes are less than about 1.5 bar. Examples of such arc tubes are described in U.S. Pat. Nos. 5,973,453 and 5,424,609, and European Patent Nos. 0 971 043 A2 and 0 954 007, all of which are incorporated herein by reference.
The conventional frit-sealing processes for ceramic arc tubes take place in low-pressure chambers, <1 bar, and employ resistive heating elements made of tungsten or graphite. The use of resistive heating necessitates bulky feedthroughs to accommodate the high electrical currents, complicated shielding, and forced water cooling. As a result, the conventional production equipment is usually large, slow, expensive and inefficient. The large sealing chambers also require larger volumes of buffer gas which increase manufacturing costs. In addition, a majority of heating energy is consumed by the apparatus itself which extends the time needed to reach the sealing temperature. The heat loss problem is exacerbated further when dealing with high buffer gas pressures because of the extra heat losses due to gas convection and increased heat transfer. Thus, there are a number of difficulties which must be overcome to obtain a ceramic arc tube having a high buffer gas pressure, i.e., >1 bar.
In contrast to ceramic arc tubes, fused silica (quartz) arc tubes have been employed with buffer gas pressures as high as 8 bar. In order to meet the high pressure requirement, a freeze-out technique is usually employed wherein one end of the quartz arc tube is immersed in liquid nitrogen to liquify or solidify the buffer gas in the discharge volume while the other end is heated to a high temperature which softens the quartz and allows the end to be sealed by a press-sealing or tipping-off method. Upon warming to room temperature, the buffer gas evaporates into a much smaller volume to provide the desired pressure. However, the freeze-out technique is impractical to use with ceramic arc tubes since the press-sealing or tipping-off methods used to seal the ends of quartz arc tubes are unavailable for use with ceramic materials.